In a wireless Positive Train Control (PTC) system, a fast-travelling locomotive communicates with a wayside or track-side base station through a radio link. A spectrum at 220 MHz has been allocated for the wireless PTC application to provide a reliable communication link between a locomotive and base stations. It is well known for digital radio systems that there are various channel impairments due to noise, multipath fading and the time-varying channel. A manifest of the net effects of the later two channel impairments is fast and deep fading, where the signal strength of received signal may be attenuated substantially in a very short period of time within the same packet. The attenuation may be more than 25 dB within 1 ms time during deep fading and the normal operation of the communication link may be temporarily interrupted.
In the PTC system, the weather damage and vandalism are the most frequent reasons that cause the service interruption since the antennas are usually installed outdoors. Often times, railroad uses dual antennas to overcome the performance problem by using an antenna splitter. Without careful impedance matching for cables, this mechanism usually causes serious performance degradation. The circuit block according to the present invention can optimize the system performance and provides matched impedance, maximizes the transmit power and receive sensitivity, and maintains the reliability of dual antenna systems against weather damage. With minimal cost added by the invented circuit, it allows to reuse existing dual antennas tower to achieve optimal performance.
There are several methods to overcome the fast fading issue. Block-based channel coding is one method that may be used to correct short burst errors associated with the fast fading. Nevertheless, the technique may require a block size sufficiently large to correct the burst errors and the large block size may not be desirable due to long latency and large memory required to store the block. Specifically, in a fast fading channel, smaller block size is highly preferable. On the other hand, multiple parallel receivers may also be used, where one receiver may be subject to deep fading at a time while the other one may still receive a good signal. However, the use of multiple parallel receivers will increase system cost. The circuit block incorporating an embodiment according to the present invention allows the receivers to use a secondary receive antenna when the primary antenna enters deep fade. In a time division multiple access (TDMA) channel, it is advantageous to configure the separate transmit and receive antennas as multiple receive antennas during the receiving period to overcome the fast fading problem.